An electrolyte analyzer is applied to determine the quantity of a specific ion component such as sodium ions, potassium ions, and chlorine ions, which are contained in a biological sample (referred to as a sample below) such as blood and urine. The concentration of specific ions in a biological fluid has a close relationship with a metabolic reaction of a living body. Various diagnoses such as hypertension symptoms, kidney diseases, and nerve disorders can be performed by measuring the concentration of specific ions.
As one of methods used in detection of ions in a sample, a method of measuring the amount of a specific ion in a sample by using an ion sensor has been known. In such a measurement using an ion sensor, first, a standard sample of a known concentration is measured in advance, and a calibration curve is calculated from a measurement result. A potential difference between an internal standard fluid and the sample is measured and the concentration of the specific ion in the sample is measured by using the potential difference and the calibration curve. In the ion sensor, the Nernst's expression (Expression 1) is established between an activity a of an ion which is a target and a potential E indicated by the ion sensor.E=E0+2.303[RT/(ZF)] log(a)  Expression 1
In Expression 1, R indicates a gas constant, and T indicates an absolute temperature. Z indicates a valency, and F indicates the Faraday constant. E0 indicates a standard electrode potential. If such an ion sensor is used, measurement of a potential enables determination of the quantity of the ion concentration.
As an anion sensor used in measurement of the chlorine ions, a solid membrane electrode using Ag/AgCl, a polymeric support liquid membrane electrode, or the like is used. The polymeric support liquid membrane electrode is obtained by holding a sensitive substance (ligand) in a polymeric support membrane formed of polyvinyl chloride and the like. The sensitive substance is formed from an organic compound. However, the latter sensor is generally used in the measurement of the chlorine ion in a biological fluid.
In the anion sensor, an anion exchange membrane is used. The anion exchange membrane uses an anion exchange resin film which contains a quaternary ammonium salt as an anion exchange group using a polymer as a basic skeleton. Selectivity for an anion in the quaternary ammonium salt depends on resolvability of an ion between the ion exchange membrane and a sample solution. A sequence of the selectivity uses a so-called Hofmeister series which relates to the hydration ability of ions, as a standard. For example, there is a problem in that if anions having high lipophilicity are present together, obtaining a measurement value of the chlorine ion is easily obstructed. Conventionally, many methods for improving anion selectivity have been proposed.
For example, PTL 1 discloses a method of performing coating treatment on a surface of the ion exchange membrane, which does not come into contact with the sample, with a two-liquid mixture-based epoxy resin, in order to prevent degradation of performance of a chlorine ion sensor.
PTL 2 discloses a method of performing manufacturing by using a manufacturing method in order to improve performance of a chlorine ion-sensitive membrane. The manufacturing method has a first process and a second process. In the first process, an anion exchange resin film is immersed into a solution obtained by dissolving meta-phenylenediamine in a solvent. In the second process, the anion exchange resin film impregnated with meta-phenylenediamine is immersed in a liquid mixture of formaldehyde and hydrochloric acid so as to generate a condensate of meta-phenylenediamine and formaldehyde.
PTL 3 discloses a method for improving preservability of a solution for an ion-selective electrode by containing an aminoglycoside antibiotic material in order to improve storing performance for consumable supplies in an ion sensor system.
PTL 4 discloses a diluted solution for an electrolyte analyzer, in which proliferation of bacteria is suppressed by adjusting a salt concentration and thus an antimicrobial effect is shown in the long term.